Tire pressure monitoring systems (TPMS) are used to detect the air pressures in each of a plurality of tires mounted on vehicle wheels and issue a warning or the like when an abnormal air pressure is detected. JP 2010-221768A discloses a tire air pressure alert system including: a sensor unit including air pressure sensors provided in each of the tires; a vehicle body-side controller that receives detection signals from the sensor unit; and an initiator (LF antenna), provided in the tire housing of each tire, that transmits a LF (low frequency) signal to the sensor unit. In a TPMS configured in this manner, the LF signals are transmitted sequentially from the tire initiators to the corresponding sensor units in response to instructions from the controller, the sensor units that have received the LF signals respond with RF (radio frequency) signals to the controller, and the controller associates the tires with the detection result. There is a problem in that the LF signals transmitted from the initiators at this time may be received not by the corresponding sensor unit, but by the sensor unit of another tire, resulting in crosstalk that makes it unclear which tire sensor unit the response signal is from. As a preventative measure against the occurrence of crosstalk, JP 2010-221768A proposes a particular configuration in which the initiators corresponding to the other sensor units, aside from the one sensor unit to be communicated with, emit interfering waves to prevent the LF signal that is to be received by the one sensor unit from being received by other sensor units.
In the system disclosed in JP 2010-221768A, it is necessary to transmit interference waves, which wastefully consumes power. A special circuit for emitting the interference waves is also necessary. Thus what is needed, in TPMS, is a way of more efficiently and accurately identifying the detection signals from tires, including preventing erroneous recognition after the mounted tires have been rotated or the like.
Having been achieved in light of the foregoing circumstances, an object of the present disclosure is to provide a tire air pressure detection system and a vehicle body side device which are capable of efficiently and accurately identifying tires and detecting the respective air pressures thereof.